Techniques relating to a heat generating element constructed by uniting a heat generative composition containing an oxidizable metal and a water-absorbent sheet are known. For example, patent literature 1 below proposes a heat generating element having an ink-like or creamy heat generative composition disposed on and sealed in an enclosing material of sheet form, the enclosing material partly possessing air permeability and water absorbency, and the enclosing material having absorbed part of the water content of the ink-like or creamy heat generative composition. Excessive water, free water, and/or water-containing gel present in the heat generative composition perform a function as a barrier layer against air and prevents an exothermic (heat generating) reaction from occurring. Excess water and the like being absorbed by the enclosing material having water absorbing properties, the barrier layer disappears thereby to cause heat generation to proceed. The patent literature 1 teaches that the heat generating element disclosed is obtained by depositing, on a base sheet (an enclosing material), a layer of an ink-like or creamy heat generative composition, the heat generative composition being prepared by stirring activated carbon, a thickener, a surfactant, a pH adjuster, edible salt, and iron powder in the order described in a prescribed compounding ratio and further kneading the mixture while adding water thereto.
Apart from that technique, patent literature 2 discloses a heat generating element including a layer of a flowable viscous heat generative composition disposed on and enclosed in an enclosing material of sheet form. The heat generating element further includes an air-permeable water absorbent sheet that covers one or both sides of the heat generative composition but is absent in the sealed portion of the enclosing material. The enclosing material of sheet form partly has air permeability. The air permeable, water absorbent sheet is fixed to a predetermined portion of the heat generative composition by the adhesive force of the heat generative composition. The heat generating element is produced by applying the flowable viscous heat generative composition to the air permeable water absorbent sheet in a prescribed pattern, overlaying another water absorbent sheet thereon to cover the heat generative composition, immobilizing the two absorbent sheets by the adhesive force of the heat generative composition sandwiched in between to make a laminate, die-cutting the laminate in a shape greater than the shape of the heat generative composition and excluding the peripheral portion that will be a sealed portion (not so great as to include the peripheral portion that will be a sealed portion), sandwiching the cut laminate between a base sheet and a cover sheet, and fusion bonding the peripheral portions of the base sheet and the cover sheet to form a sealed portion.
The assignee common to this application previously disclosed in patent literature 3 below a method for producing a heat generating molded article. The method includes applying a coating fluid containing oxidizable metal powder, a fibrous material, water, and a moisture retaining agent and having a water content of 40% to 75% by mass to a support to form a water-containing molded mat, dewatering the water-containing molded mat to a prescribed water content, heat drying the dewatered water-containing molded mat to a prescribed water content to make an intermediate product, and incorporating a prescribed amount of an aqueous electrolyte solution into the resulting intermediate product to make a heat generating molded article. According to this method, since the coating fluid is free from an electrolyte, oxidation of the oxidizable metal particles is prevented from proceeding during the steps of applying the coating fluid, dewatering, and drying to make the intermediate product, and the disperse state of the heat generative composition is maintained.
Patent literature below 4 describes a heat generating sheet comprising a heat generative composition and a support, the support being formed of nonwoven fabric and supporting the heat generative composition. The heat generative composition is held in a great number of voids in the nonwoven fabric. As proposed by the literature, the heat generative composition may be held by a method including spreading a mixture of powdered raw materials, such as iron powder, activated carbon, and an inorganic electrolyte, on nonwoven fabric and vibrating the nonwoven fabric to allow the powdered raw materials to penetrate into the voids inside the nonwoven fabric.
Patent literature 5 discloses a process of producing a heat generating molded article including the steps of making an intermediate product by a papermaking process from a raw material composition containing an oxidizable metal powder, a moisture retaining agent, a fibrous material, and water and incorporating an electrolyte into the resulting intermediate product. The electrolyte can be incorporated into the intermediate product by impregnating the intermediate product with a solution of the electrolyte having a prescribed concentration or adding the electrolyte in a solid state with a prescribed particle size to the intermediate product.